Variations in strength-related measures during the menstrual cycle in eumenorrheic women: A systematic review and meta-analysis

Abstract

Objectives

To systematically review the current body of research that has investigated changes in strength-related variables during different phases of the menstrual cycle in eumenorrheic women.

Design

Systematic review and meta-analysis.

Methods

A literature search was conducted in Pubmed, SPORTDiscus and Web of Science using search terms related to the menstrual cycle and strength-related measures. Two reviewers reached consensus that 21 studies met the criteria for inclusion. Methodological rigour was assessed using the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. Random effects meta-analyses were used to compare the early-follicular, ovulatory and mid-luteal phases for maximal voluntary contraction, isokinetic peak torque, and explosive strength.

Results

The assessment of study quality showed that a high level of bias exists in specific areas of study design. Non-significant and small or trivial effect sizes (p ≥ 0.26, Hedges g ≤ 0.35) were identified for all strength-related variables in each comparison between phases. 95% confidence intervals for each comparison suggested the uncertainty associated with each estimate extends to a small effect on strength performance with unclear direction (−0.42 ≤ g ≤ 0.48). The heterogeneity for each comparison was also small (p ≥ 0.83, I² = 0%).

Conclusions

Strength-related measures appear to be minimally altered (g ≤ 0.35) by the fluctuations in ovarian sex hormones that occur during the menstrual cycle. This finding should be interpreted with caution due to the methodological shortcomings identified by the quality assessment.

Introduction

The menstrual cycle represents an important biological rhythm in females that serves to prepare the uterus for gestation. In eumenorrheic women, a menstrual cycle typically lasts 28 days, but can vary considerably.¹ It is well-established that over the course of a menstrual cycle, women are exposed to a constant and rapidly shifting profile of endogenous sex hormones. Aside from their principal roles in reproductive function and control of sexual characteristics, the main female ovarian hormones (oestrogen and progesterone) circulating in the blood, influence a multitude of different physiological systems. Fluctuations in these, and other sex hormones, can explain variations in physical performance and physiological responses to exercise over the course of a menstrual cycle,² which has important implications for scientific research and the optimization of exercise prescription in females. In particular, the effect that changes in female reproductive hormones exert in strength-related tasks has received considerable attention and is widely debated.2, 3, 4, 5

The menstrual cycle is traditionally divided into two distinct phases (follicular and luteal), which are separated by ovulation. These two phases are defined by ovarian function and differentiated by varying concentrations of oestrogen and progesterone. The follicular phase begins on the first day of menses (days 1–5) and is characterized by low concentrations of both oestrogen and progesterone. Oestrogen gradually increases during the follicular phase and peaks ∼1 day prior to ovulation (typically 12–14 days after menstruation onset), which is triggered by a surge in the luteinizing hormone (LH). The rise in oestrogen and LH is also accompanied by a sharp and brief increase in testosterone, which is a precursor for the biosynthesis of oestrogen, and is considered to be important for sexual function and desire in females.⁶ Following the ovulatory period, the early-luteal phase is characterized by decreasing oestrogen levels and a gradual rise in progesterone. Through the mid-luteal phase, oestrogen displays a bi-phasic response, resulting in high levels of both hormones before a gradual decrease over the following 5–7 days. Although this pattern of hormonal physiology is broadly present in all eumenorrheic females, the concentration of hormones and timing of cycle events, displays large inter-individual variability,⁷ making scientific investigation in this area complex and challenging. Therefore, although many studies have investigated the effects of menstrual cycle phase on exercise performance, it is prudent to examine the internal validity of these studies to ensure that the conclusions generated are accurate.

Oestrogen is responsible for the regulation of a number of important anabolic processes⁸ and can influence central nervous system function.⁹ Dehydroepiandrosterone (DHEA), the precursor to oestrogen and testosterone that peaks prior to ovulation, produces a net excitatory effect via its action on several neurotransmitter receptors.¹⁰ Specifically, oestrogen is known to bind to receptor sites that attenuate the release of γ-aminobutyric acid,¹¹ a neurotransmitter responsible for reducing neuronal excitability and muscle tone. Additionally, oestrogen promotes the activation of glutamate releasing neuron receptors that cause an excitatory response in the nervous system.¹² In contrast, progesterone is known to exert a net inhibitory effect on the nervous system via enhancement of γ-aminobutyric acid action.¹³ Variations in testosterone level across the menstrual cycle also produce physiological effects that may alter strength-related performance. Although absolute testosterone is low in females compared to males, a surge in testosterone during the late-follicular phase may benefit performance during short-intense activity, via increases in motivational drive,¹⁴ and enhanced calcium kinetics in the muscle cell.¹⁵ It is also possible that the thermogenic action of progesterone, which causes an increase in core body and skin temperature during the luteal phase,¹⁶ positively influences nerve conduction velocity¹⁷ and antagonistic co-contraction,¹⁸ and consequently may positively influence performance in explosive strength-related tasks. Based upon these theoretical mechanisms, it is conceivable that oestrogen has an inotropic effect on muscular strength-related capabilities.

Strength can be defined as the ability of an individual to apply force under a specified set of movement constraints.¹⁹ Increases in strength are associated with improvements in important markers of metabolic health,²⁰ everyday tasks,²¹ a lower risk of injury,²² and enhanced athletic performance.²³ Strength training is also important in counteracting conditions associated with muscle weakness, such as sarcopenia, musculoskeletal disorders, and prolonged immobilization.²⁴ Strength training also provides beneficial changes to the risk factors associated with anterior-cruciate ligament injury, which has a high incidence in female athletes injury.²⁵

Findings from studies examining the influence of female sex hormones on strength-related performance have previously been summarized in several non-systematic narrative reviews and book chapters.1, 2, 3, 4, 26 However, these works typically address the effects of female reproductive hormones on athletic performance qualities more broadly and concluded that the effects on muscular strength are equivocal. Consequently, the aim of this study was to systematically review the current body of research that has investigated changes in strength-related variables during different phases of the menstrual cycle in eumenorrheic women, and to conduct a meta-analysis of the data.